In the one when Rachel tell Ross that "There is no us", at the end when Monica is going through her old boxes her parents dropped of, she finds a prom video, then Ross comes in and doesn't want to watch it. When the video is over it shows the TV go off, and in the screen of the TV you can see the reflection of Monica moving from shutting it off, but it instantly cuts to her, and she's sitting in the same spot unmoved!